In general, the intensity distribution of light emerging from optical resonators, especially those used in high power lasers with large apertures, has a multimode pattern. In this pattern, the intensity is typically distributed in spots of different sizes, smaller in diameter than the aperture of the resonator. These spots have random phases, and in an unpolarized resonator, have different polarizations. This pattern of spots fills most of the gain medium volume, and thus efficiently extracts the power of the laser. However, the multimode pattern results in an output beam of relatively low brightness, compared to the diffraction limit of the resonator. This low brightness, in turn, limits the usage of the laser beam in many industrial, medical and military applications where a small, well-defined, focused spot or a well collimated beam is needed, such as in scribing, drilling, cutting, target designation and rangefinding.
Ever increasing attempts are being made to design laser resonators which emit a beam of high brightness and high power, i.e. lasing with a single low order mode that fills and utilizes most if not all of the gain medium. This goal is difficult to achieve in a resonator with a high Fresnel number, i.e. with a wide aperture and short length, such as is desired for optimally compact lasers. In these resonators, there is hardly any loss discrimination between the different modes of oscillation and these resonators thus emit high divergence multimode beams. Particular examples of such resonators are those used in CW and pulsed solid state lasers.
A common approach used to control the modes of a resonator is to introduce an aperture inside the resonator. The aperture causes loss to higher order modes thus limiting their oscillation, so they virtually cease to exist. By use of the correct aperture, a laser can be made to emit in the fundamental mode TEM00, which possesses the highest brightness of all possible modes. However, in a high Fresnel number resonator, this mode does not fill the entire gain medium diameter, resulting in poor efficiency of the laser.
A number of other methods have been proposed to obtain a specific, stable mode in a large lasing volume. For example, in the article “Single-mode selection using coherent imaging within a slab waveguide CO2 laser” published in Applied Physics Letters, Vol. 60, pp. 2469-2471 (1992), K. M. Abramski, H. J. Baker, A. D. Colly and D. R. Hall propose the insertion of a wire grid into the laser resonator for selecting a specific high order mode. Other methods are reviewed and discussed in the co-pending patent applications “Optical Resonators with Discontinuous Phase Elements”, Application No. PCT/IL98/00204, Publication No. WO98/50986 and “Optical Resonators with Spiral Phase Elements” Application No. PCT/JL97/00064, Publication No. WO97/34344. As discussed in the prior art, these methods have some performance limitations, or have some difficulties in practical implementation, such as an inability to sufficiently extract power from the gain medium.
One of the methods proposed in the above-mentioned patent applications has recently been demonstrated by R. Oron et al. in the article “Discontinuous phase elements for transverse mode selection in laser resonators” in Applied Physics Letters, Vol. 74 (10), pp. 1373-1375, (1999). In this article, a method is described for causing a resonator to oscillate in a single mode, which need not necessarily be the fundamental TEM00 mode. This method involves introduction of phase elements, either discontinuous or continuous, into the resonator. The phase distribution of the phase elements impose different losses to different modes, thus discriminating between them, since only the mode with a phase distribution that matches that of the element suffers no loss. Since the phase element makes changes only to the phase of the radiation field within the cavity, it does not introduce extra loss to the resonator, unlike discriminators that modulate the amplitude such as apertures, wires, apodizers and the like.
The resulting single high order mode distribution, in contrast to a multimode distribution, has a controlled phase, and thus is more easily focusable. In addition, since the high order mode is larger in diameter than the fundamental mode, it more fully fills the gain medium diameter, resulting in higher resonator efficiency and an output beam of higher power.
Yet, the single high-order mode still does not utilize the entire gain medium volume, since modes of order higher than the fundamental mode have both zero intensity zones (nodes) and low intensity regions. In these regions, the electromagnetic field does not create stimulated emission which is the mechanism for extracting power from the lasing medium.
There therefore exists a serious need for a method of operating the resonator of a laser more efficiently, so as to effectively utilize a larger part of the lasing medium.
The disclosures of all publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety.